This invention relates to a novel liquid crystal display device making use of a liquid crystal material having chiral smectic C phase.
As the tendency of development of liquid crystal display, multiplex driving of the STN mode or TFT active matrix driving of the TN mode has been predominant. However, for achieving a larger screen format or a higher definition, a device operating with a high speed becomes necessary.
Under this situation, a ferroelectric liquid crystal having the chiral smectic C phase is attracting attention as the liquid crystal material employed in the liquid crystal display devices. As the ferroelectric liquid crystal device, employing the ferroelectric liquid crystal and taking advantage of the light switching effects of the chiral smectic C phase, there is known a surface-stabilized ferroelectric liquid crystal (SSFLC) device, as proposed by N. A. Clark and S. T. Lagawall.
The SSFLC device is a liquid crystal device in which a liquid crystal component exhibiting the chiral smectic C phase (SmC* phase) is charged into a narrow gap cell. With the SSFLC device, the stable state is one in which axes of the liquid crystal molecules are aligned in a direction inclined by predetermined angles .alpha. and -.alpha. with respect to the direction of processing for substrate orientation. Bright and dark states are caused to appear by switching between these two bistable states.
Thus a problem is raised with SSFLC that it can be switched only between the two states of the bright and dark states and that, while it may function as a memory, it is unable to perform gray-scale display.
As a technique for achieving gray-scale display in a ferroelectric liquid crystal device, there has been proposed a method of controlling multidomains with different threshold voltages for achieving areal gradation. However, this method has a deficiency that the threshold values of the domains are discrete and continuous gray-scale cannot be achieved.
For overcoming the deficiency, a liquid crystal display cell has been proposed in Japanese Patent KOKAI publication 1-152430(1989), according to which an SmC* liquid crystal in which the relation between the pitch P and the cell gap d of the SmC* phase is given by d/p&gt;5, that is in which p&lt;0.4 .mu.m for d=2 .mu.m, is employed, the helical axis of the SmC* phase is aligned with the direction of processing for substrate orientation and the director (molecular axis) of the liquid crystal molecule forms a helix in the stable state. With this liquid crystal display cell, analog gray-scale of the television rate may be achieved by low voltage driving under the monostable state employing the effect of helical distortion of the ferroelectric liquid crystal.
However, in this case, a new problem is raised that a liquid crystal system with an extremely short helical pitch becomes necessary because of the fast response characteristics and the operating principle thereof, such that it is difficult to achieve uniform orientation.
Thus it is difficult with the conventional ferroelectric liquid liquid crystal device to achieved continuous gray-scale display (so-called analog gray-scaleal display) so that its practical usage is limited significantly.
In view of the above described state of the art, it is an object of the present invention to provide a liquid crystal display device in which continuous gray-scaleal display is feasible and in which orientation of the liquid crystal may be achieved easily.